Hydrocarbon radical cations, structure and

Hydrocarbon radical cations, structure and reactivity of, 38, 87 Hydrocarbons, small-ring, gas-phase pyrolysis of, 4, 147... 

The mechanistic aspects of aromatic121 and alkene122 radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy.123 A review of the reactivity of divalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics.124 The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms.122125... 

Structure, determination of organic reactivity, 35, 67 Structure and mechanism, in curbene chemistry, 7, 153 Structure and mechanism, in organic electrochemistry, 12, 1 Structure and reactivity of carbencs having aryl substitutents, 22, 311 Structure and reactivity of hydrocarbon radical cations, 38, 87 Structure of electronically excited molecules, 1, 365... 

The majority of radical cations identified and characterized to date are relatively stable and their structures are closely related to those of the neutral diamagnetic precursors. In particular, a large number of species derived from aromatic hydrocarbons has been characterized by ESR [3] and optical spectroscopy [4], The close structural similarity manifests itself in an interesting relationship between the UV spectra of selected radical cations and the UV photoelectron spectra of their parent molecules. Since both transitions lead to the same (excited) state of the radical cation, the excitation energies, AE, of the radical cation correspond to differences in ionization energies, AI, documented in the photoelectron spectroscopic data of the parent molecules [7, 276, 277],... 

We will approach radical cation structures according to the nature of the parent molecules, specifically according to the donor type, viz., n-, or o-donors, to which they belong. Among the radical cations derived from rc-donors, those of aromatic hydrocarbons show the closest structural relationship to their parents. They also were the first class to be investigated in detail, because they are comparably stable and their spectra fall into a readily accessible range. This family shows the closest correlation between radical cation AEs, and parent AIs. On the other hand, cross-conjugated systems and alkenes may feature substantial differences between parent and radical cation electronic structures. Hence their tendency towards non-Koopmans type states. 

Both we and others have established various radical cation structure types, which deviate in important features from the structures of their neutral diamagnetic precursors. The pursuit of these novel structure types has given new direction to radical cation chemistry. We have noted that some of these species resemble plausible transition structures for the thermal rearrangement of the parent molecules, i.e. saddle points on the corresponding potential surfaces. From a different point of view, they can be envisaged as one-electron oxidation products of biradicals or zwitterions. However, this relationship rarely serves as a practical approach to their generation, since the potential bifunctional precursors are often unstable and not readily accessible. These radical cations are usually generated from related hydrocarbons or cyclic azo compounds. 

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

Quadricyclanes (160) also undergo a valence isomerization to norborna-dienes if irradiated in the presence of electron acceptors such as fumaro-nitrile (Jones and Becker, 1982). Two distinct radical cation structures are observed for the hydrocarbon, corresponding roughly to the bonding patterns of norbornadiene and quadricylane, respectively (Roth et al., 1981). 

The implications of these unexpected results for the conclusions drawn from previous studies, in particular the work based on spectroelectrochemical measurements, are not clear. The structure of the dimer dication, (ArCH )2, is not known. However, the reversible formation of dimers from hydrocarbon radical cations has been observed in other cases [203], and is probably a general phenomenon, but usually the dimers do not manifest themselves kinetically. 

Equilibrium structures and hyperfine parameters of some fluorinated hydrocarbon radical cations a DFT B3LYP and MP2 study73... 

---